Priority is claimed to Japanese Patent Application No. 10-182524 filed in Japan on Jun. 29, 1998, the entire contents of which is hereby incorporated by reference.
1. Field of the Invention
This invention relates to a voltage controlled oscillation circuit, and particularly relates to a voltage controlled oscillation circuit used for mobile communication apparatuses, such as a portable telephone.
2. Description of the Related Art
With a size-reduction of mobile-communication apparatuses, such as portable telephones, the demand of a size-reduction of RF circuits including circuits such as voltage-controlled oscillation circuits and PLL circuits has become strong in recent years.
The circuit diagram of a conventional voltage controlled oscillation circuit is shown in FIG. 6. The voltage controlled oscillation circuit 1 comprises an amplifier circuit 2, a resonance circuit 3, three capacitors C1, C2 and C3, a varactor diode D1, and a choke element L1. The amplifier circuit 2 is connected to the resonance circuit 3, and the varactor diode D1 is connected in parallel to the resonance circuit 3 via a first capacitor C1. One end of the choke element L1 is connected to the cathode of the varactor diode D1, and the other end of the choke element L1 is connected to the ground via a third capacitor C3. Moreover, a second capacitor C2 is connected in parallel to the choke element L1. The output of the amplifier circuit 2 is connected to an output terminal 4, and the other end of the choke element L1 is connected to a control voltage terminal 5.
The control voltage terminal 5 is a terminal for changing the oscillation frequency of the voltage controlled oscillation circuit 1. A DC voltage is applied to the varactor diode D1, the internal capacitance of the varactor diode D1 is changed. The changes of the internal capacitance brings the changes of the resonance frequency of the resonance system including the resonance circuit 3, the varactor diode D1 or the like. As a result, the oscillation frequency of the voltage controlled oscillation circuit 1 varies accordingly. Moreover, the choke element L1 is an inductor provided such that the load connected to the control voltage terminal 5 might not influence the resonance circuit 3. For example, the choke element L1 comprises of a microstrip line having a predetermined length and a narrow width. The third capacitor C3 is a bypass capacitor. The second capacitor 2 is provided such that the second capacitor 2 is caused to be resonated with the choke element L1 in the vicinity of the oscillation frequency and the impedance between the cathode of the varactor diode D1 and the control voltage terminal 5 in an oscillation frequency is made as high as possible. In this way the second capacitor 2 compensates the characteristics, as the choke element L1 does not necessarily work as an ideal choke element.
Usually, the control voltage terminal 5 of the voltage controlled oscillation circuit is connected to a loop filter when PLL circuit is formed. In FIG. 6, a loop filter 6 is shown. A resistor R1 and a fourth capacitor C4 are included in the output side of the loop filter 6. When the loop filter 6 is connected to the control voltage terminal 5 of the voltage controlled oscillation circuit 1, the resistor R1 built in the loop filter 6 is connected in series to the control voltage terminal 5 of the voltage controlled oscillation circuit 1. Moreover, the fourth capacitor C4 is connected in parallel to the third capacitor C3 of the voltage controlled oscillation circuit 1.
When a demand for a size-reduction of RF circuit was not excessively strong, the voltage controlled oscillation circuit 1 and the loop filter 6 were able to be arranged with a wider distance between them. In this case, siring is formed, for example, on a mounting substrate from the output of the loop filter 6 to the control voltage terminal 5 of the voltage controlled oscillation circuit 1, and this wiring works as an inductance in the high frequency corresponding to the oscillation frequency of the voltage controlled oscillation circuit 1. Thus, the fourth capacitor C4 in the output side of the loop filter 6 did not necessarily affect the characteristic of the voltage controlled oscillation circuit 1.
However, when demand for a size-reduction of RF circuit becomes strong and the distance of the voltage controlled oscillation circuit 1 and the loop filter 6 becomes small, the inductance component of the wiring between them is almost eliminated, and the fourth capacitor C4 in the output side of the loop filter 6 comes to be connected in parallel to the third capacitor C3 of the voltage controlled oscillation circuit 1.
At this time, a sub-resonance (i.e., a secondary resonance as a by-product) occurs in the resonance circuit of the voltage controlled oscillation circuit 1 by that the fourth capacitor C4 is connected in parallel to the third capacitor C3. Particularly in order to improve a lock-up time, when the capacitance of the fourth capacitor C4 in the output side of the loop filter 6 is made small, the frequency of the sub-resonance is generated in the vicinity of the original oscillation frequency of the voltage controlled oscillation circuit 1. Conversely, when the distance of the voltage controlled oscillation circuit 1 and the loop filter 6 is made large, it means that the frequency of the sub-resonance is located far away from the original oscillation frequency since the inductance component of the wiring between the voltage controlled oscillation circuit 1 and the loop filter 6 is large.
In FIG. 7, the impedance of a resonance circuit 3 side ( mag (Z11), abbreviation of magnitude (Z11), logarithmic representation of the absolute value of Z11) is shown. The resonance circuit 3 side is viewed from the node of the amplifier circuit 2 of the voltage controlled oscillation circuit 1, and the resonance circuit 3 side includes all of the varactor diode D1, the capacitor C4 or the like.
As shown in FIG. 7, though the pole of the impedance (resonance point) is in the original oscillation frequency f1, another pole of the impedance (sub-resonance point) exists also in the frequency f2 which is a little lower than the frequency f1. Due to this sub-resonance point, when the oscillation frequency of the voltage controlled oscillator 1 is changed, there is a problem that an abnormal oscillation depending on conditions occurs.
In FIG. 8, the relationship of a control voltage (DC voltage applied to the control voltage terminal 5) and an oscillation frequency is shown when the fourth capacitor C4 is connected in parallel to the control voltage terminal 5 of the voltage controlled oscillation circuit 1. As is clear from FIG. 8, the abnormal oscillation occurs when the control voltage exceeded 2.6V, and a normal oscillation stops. When the control voltage is 2.8V, the oscillation resumes at the different frequency far away from the oscillation frequency whet the control voltage is 2.6V or less. When the control voltage is 2.6V or less, the oscillation frequency is a range from approximately 700 MHZ to 740 MHZ, on the other hand, when the control voltage is 2.8V, the oscillation frequency is approximately 820 MHZ. Moreover, changes of the oscillation frequency to changes of the control voltage also becomes small.
It is an object of the present invention to solve the above-described problems, and to provide the voltage controlled oscillation circuit preventing the influence due to the capacitor in the output side of the loop filter connected to a control voltage terminal.
In order to achieve the above-mentioned object, the present invention is to provide a voltage controlled oscillation circuit including an amplifier circuit, a resonance circuit connected to the amplifier circuit, a varactor ode connected to the resonance circuit, one end of a choke element connected to the cathode of the varactor diode, a capacitor connected between the other end of the choke element and a ground, and an inductance element or a resistive element connected to the other end of the choke element.
Due to the above configuration, the voltage controlled oscillation circuit of the present invention is not affected by the capacitor in the output side of the loop filter connected to the control voltage terminal.